1. Field of the Invention
The present invention relates to an electrophotographic photoconductor in which a charge generation layer and a charge transport layer are successively provided on an electroconductive support. In addition, the present invention relates to an electrophotographic image forming apparatus using the above-mentioned photoconductor and a light source with a wavelength in the range of 400 to 450 nm as a light exposure means for data recording. The present invention also relates to a process cartridge including the photoconductor, which process cartridge is freely attachable to the image forming apparatus and detachable therefrom.
2. Discussion of Background
It is well known that a photoconductor for use with an electrophotographic process employs a photoconductive material, which is divided into an inorgnaic photoconductive material and an organic photoconductive material.
According to the above-mentioned electrophotographic process, image formation is usually achieved by following the procedures shown below. The surface of a photoconductor is uniformly charged in the dark, for example, by corona charging, and exposed to light images to selectively dissipate electric charge of a light-exposed portion, thereby forming latent electrostatic images on the surface of the photoconductor. The latent electrostatic images are developed as visible toner images with a toner that is made up of a coloring agent, such as a dye or pigment, and a polymeric material. Image formation can thus be repeated, using the photoconductor, by the so-called Carlson process, for an extended period of time.
Most of the currently available photoconductors employ organic photoconductive materials. This is because an organic photoconductive material is superior to an inorganic material in terms of the degree of freedom in selection of wavelength of light to which the photoconductive material is sensitive, the filming forming properties, flexibility, transparency of the obtained film, mass productivity, toxicity, and cost.
The photoconductor repeatedly used in the electrophotographic process or the like is required to have basic electrostatic properties such as good sensitivity, sufficient charging potential, charge retention properties, stable charging characteristics, minimal residual potential, and excellent spectral sensitivity.
In recent years, data processors employing the electrophotographic process have exhibited remarkable development. The image quality and printing reliability have noticeably improved, in particular, in the field of a printer that adapts a digital recording system by which information is converted into a digital signal and recorded by means of light. Such a digital recording system is applied to not only printers, but also to copying machines. Namely, a digital copying machine has been actively developed. Further, there is a tendency for the digital copying machine to be provided with various data processing functions, so that demand for the digital copying machine is expected to rise sharply.
A function-separation layered photoconductor has become the mainstream in the field of electrophotographic photoconductors for the above-mentioned digital copying machine. The function-separation layered photoconductor is constructed in such a manner that a charge generation layer and a charge transport layer are successively overlaid on an electroconductive support. To improve the durability of the photoconductor from the mechanical and chemical viewpoints, a surface protection layer may be overlaid on the top surface of the photoconductor.
When the surface of the function-separation layered photoconductor is charged and thereafter exposed to light images, the light passes through the charge transport layer and is absorbed by a charge generation material in the charge generation layer. Upon absorbing light, the charge generation material produces a charge carrier. The charge carrier is injected into the charge transport layer and travels along an electric field generated by the charging step to neutralize the surface charge of the photoconductor. As a result, latent electrostatic images are formed on the surface of the photoconductor.
In view of the above-mentioned mechanism of the function-separation layered photoconductor, a charge generation material which exhibits absorption peaks within the range from the near infrared region to the visible light region is often used in combination with a charge transport material that does not hinder the charge generation material from absorbing light, in other words, exhibiting absorption within the range from the visible light region (yellow light region) to the ultraviolet region.
As a light source capable of coping with the above-mentioned digital recording system, a semiconductor laser diode (LD) and a light emitting diode (LED), which are compact, inexpensive, and highly reliabler are widely employed. The LD most commonly used these days has an oscillation wavelength range in the near infrared region of around 780 to 800 nm. The emitting wavelength of the typical LED is located at 740 nm.
Recently, an LD or LED with oscillation wavelengths of 400 to 450 nm to emit a violet or blue light has been developed and finally put on the market as a light source for writing information so as to cope with the digital recording system. This kind of LD or LED is hereinafter referred to as xe2x80x9cshorter wavelength LD or LED.xe2x80x9d In the case where a shorter wavelength LD, of which the oscillation wavelength is as short as nearly half the conventional one located in the near infrared light region, is used as the light source for writing, it is theoretically possible to decrease the spot size of a laser beam projected on the surface of a photoconductor, in accordance with the following formula (A):
d=(xcfx80/4)(xcexf/D) xe2x80x83xe2x80x83(A) 
wherein d is the spot size projected on the surface of the photoconductor, xcex is the wavelength of the laser beam, f is the focal length of a fxcex8 lens, and D is the lens diameter.
In other words, the use of the shorter wavelength LD or LED can enormously contribute to improvement of the recording density, that is, resolution, of a latent electrostatic image formed on the photoconductor.
Further, for the use of such a shorter wavelength LD or LED, it will be possible to make the electrophotographic image forming apparatus compact as a whole, and to speed up the electrophotographic image forming method. Accordingly, there is an increasing demand for high sensitivity and high stability of the electrophotographic photoconductor so as to cope with the light source of the LD or LED having wavelengths of about 400 to 450 nm.
As previously mentioned, the function-separation layered photoconductor has been the mainstream of the electrophotographic photoconductors. With such a layered structure, the charge transport layer is usually overlaid on the charge generation layer. High sensitivity can be obtained if light emitted from the shorter wavelength LD or LED can efficiently reach the charge generation layer after passing through the charge transport layer. Namely, it becomes important that the charge transport layer not absorb the light from the LD or LED.
The charge transport layer is generally a film with a thickness of about 10 to 30 xcexcm made from a solid solution in which a low-molecular weight charge transport material is dispersed in a binder resin. Most of the currently available photoconductors employ as a binder resin for the charge transport layer a bisphenol polycarbonate resin or a copolymer consisting of a monomer of the above-mentioned polycarbonate resin and any other monomers. The bisphenol polycarbonate resin has the characteristics that no absorption appears in the wavelength range from 390 to 460 nm. Therefore, the bisphenol polycarbonate resin does not severely hinder the light for a recording operation from passing through the charge transport layer.
The following are commercially available charge transport materials that are conventionally known: 1,1-bis(p-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene (Japanese Laid-Open Patent Application 62-30255), 5-[4-(N,N-di-p-tolylamino)benzylidene]-5H-dibenzo[a,b]cyclo-heptene (Japanese Laid-Open Patent Application 63-225660), and pyrene-1-aldehyde 1,1-diphenylhydrazone (Japanese Laid-Open Patent Application 58-159536). These conventional charge transport materials exhibit absorption in the wavelength range of 390 to 460 nm. Therefore, the light emitted from the above-mentioned shorter wavelength LD or LED is unfavorably absorbed in a surface portion of the charge transport layer. As a result, the light cannot reach the charge generation layer, whereby the photosensitivity cannot be obtained in principle.
Japanese Laid-open Patent Applications 55-67778 and 9-190054 state that when light with a particular wavelength which will be absorbed by the charge transport material is used, a decrease in charging characteristics and an increase in residual potential are caused during repeated operations. Light absorption by the charge transport material lowers the photosensitivity, and in addition, has an adverse effect on the fatigue behavior in the repetition.
There are some charge transport layers that can exhibit high sensitivity when used in the layered photoconductor although the short-wavelength light can hardly pass through those charge transport layers. The mechanism of this phenomenon is disclosed in Japanese Laid-Open Patent Application 5-61216 and Japan Hard Copy ""91, p. 165. Namely, when the charge transport material absorbs light that is projected on the photoconductor for data recording, the behavior of the charge transport material is as follows: after the charge transport material is first optically excited, the charge transport material fluoresces light of which wavelength is longer than the light projected on the charge transport layer, and thereafter the charge transport material becomes inactivated. The fluorescence emitted from the charge transport material is partially dissipated from the surface of the photoconductor, but mostly trapped in the photoconductor. The fluorescence trapped in the photoconductor repeatedly causes multiple reflection in the photoconductive layer until the fluorescence is absorbed by a charge generation material. Further, unfavorably, such fluorescence occurs in a surface portion of the charge transport layer, and light advances in every direction. The result is that a latent image formed on the photoconductor shows a decreased resolution, thereby inducing image blur.
It is known that light absorption by the charge transport material has an adverse effect not only on the sensitivity, but also the fatigue characteristics caused by repeated operations and the resolution of a latent image. Japanese Laid-Open Patent Application 12-105471 discloses an electrophotographic photoconductor that can cope with a light exposure means using a light source with a short wavelength. A charge transport layer of the photoconductor exhibits light transmitting properties of 30% or more with respect to the above-mentioned light with short wavelengths. Such high light transmitting properties of the charge transport layer can effectively increase the sensitivity of the photoconductor. However, in the case where the charge transport layer shows not only high light transmitting properties, but also a large fluorescence generation efficiency, the resolution of latent images formed on the photoconductor is lowered, as previously mentioned. Most of the charge transport materials disclosed in the above-mentioned application considerably absorb light projected on the photoconductor for the formation of latent images, so that there is a serious problem in the repetition stability.
Japanese Laid-Open Patent Application 12-89492 discloses the use of a charge transport material with a quantum yield of 0.1 or more. Disadvantageously, however, the resolution of latent images formed on the photoconductor is similarly lowered.
Japanese Laid-Open Patent Application 9-240051 discloses an electrophotographic image forming apparatus which employs as a light source an LD beam With an oscillation wavelength of 400 to 500 nm. An electrophotographic photoconductor for use in the above-mentioned image forming apparatus is constructed in such a manner that a charge transport layer and a charge generation layer are successively overlaid on an electroconductive support in that order to aim at high resolution of the obtained image. However, the charge generation layer in the form of a fragile thin film is exposed to mechanical and chemical hazards in the cycle of charging, development, image transfer, and cleaning steps. The photoconductor deteriorates too badly to be used in practice.
The above-mentioned Japanese Laid-Open Patent Application 9-240051 also discloses an electrophotographic photoconductor of a single-layered structure, This kind of photoconductor has the problems that design of the constituent materials is limited and the sensitivity cannot increase as high as that of the function-separation layered photoconductor.
In the field of the electrophotographic image forming apparatus such as printers and copying machines, the diameter of a photoconductor tends to decrease in line with the development of high-speed operation, small-size apparatus, and high-quality image formation. This tendency makes the operating conditions of the photoconductor much more severe in the electrophotographic process.
For example, a charging roller and a cleaning rubber blade are disposed around the photoconductor. An increase in hardness of the rubber and an increase in contact pressure of the rubber blade with the photoconductor become unavoidable to obtain adequate cleaning performance. As a result, the photoconductor suffers from wear, and therefore, the potential and the sensitivity of the photoconductor are always subject to variation. Such variation produces abnormal images, impairs the color balance of color images, and lowers the color reproducibility.
In addition, when the photoconductor is operated for a long period of time, ozone generated in the course of the charging step oxidizes a binder resin and a charge transport material. Further, ionic compounds such as nitric acid ion, sulfuric acid ion, and ammonium ion, and organic acid compounds generated in the charging step are accumulated on the surface of the photoconductor, which will lead to great deterioration of image quality.
In light of the above, it is considered important to upgrade the durability of the photoconductor and improve the physical properties of the top surface layer of the photoconductor.
It is therefore a first object of the present invention to provide an electrophotographic photoconductor which can exhibit high sensitivity to a light source such as a laser diode (LD) or light emitting diode (LED) with a wavelength in the range of 400 to 450 nm, and excellent stability during the repeated operations.
A second object of the present invention is to provide a process cartridge holding therein the above-mentioned photoconductor.
A third object of the present invention is to provide an electrophotographic image forming apparatus including the above-mentioned photoconductor.
The first object of the present invention can be achieved by an electrophotographic photoconductor comprising an electroconductive support, a charge generation layer formed thereon, and a charge transport layer formed on the charge generation layer, the charge transport layer allowing any monochromatic light with wavelengths of 390 to 460 nm to pass, and the charge transport layer exhibiting a fluorescence generation efficiency of 0.8 or less when irradiated with the above-mentioned monochromatic light.
The second object of the present invention can be achieved by a process cartridge which is freely attachable to an electrophotographic image forming apparatus and detachable therefrom, the process cartridge holding therein the above-mentioned electrophotographic photoconductor, and at least one means selected from the group consisting of a charging means for charging a surface of the photoconductor, a light exposure means for exposing the photoconductor to a light image to form a latent electrostatic image on the photoconductor, a development means for developing the latent electrostatic image to a visible image, an image transfer means for transferring the visible image formed on the photoconductor to an image receiving member, a cleaning means for cleaning the surface of the photoconductor, and a quenching means.
The third object of the present invention can be achieved by an electrophotographic image forming apparatus comprising the above-mentioned electrophotographic photoconductor, means for charging a surface of the photoconductor, means for exposing the photoconductor to a light image to form a latent electrostatic image on the photoconductor, means for developing the latent electrostatic image to a visible image, and means for transferring the visible image formed on the photoconductor to an image receiving member.